Part I. Structure of Small Faults in a Quartz–Syenite Body, Southern Israel

Journal of Structural Geology 25 (2003) 1675–1689 www.elsevier.com/locate/jsg

Faults and their associated host rock deformation: Part I. Structure of small faults in a quartz–syenite body, southern Israel

Oded Katza,b,*, Ze’ev Rechesa, Gidon Baerb

aInstitute of Earth Sciences, Hebrew University, Jerusalem 91904, Israel bGeological Survey of Israel, 30 Malkhe Israel St., Jerusalem 95501, Israel

Received 1 September 2001; received in revised form 1 August 2002; accepted 9 August 2002

Abstract We analyze pervasive and discontinuous deformation associated with small faults in a quartz–syenite body in southern Israel. The analysis includes detailed mapping, measurement of in-situ mechanical rock properties and microstructural study of the faults. The mapped faults have 1–100-m-long horizontal traces, consisting of linked, curved segments; the segmented nature of the faults is also apparent at the 1– 10 mm scale. The observed deformation features are breccia, as well as intra- and inter-granular fractures; these features are accompanied by reduction of the Young modulus and uniaxial strength of the host rock. The deformation features are zoned from a central fault-core through a damage-zone to the protolith at distances of 0.05–0.06 the fault length. Shear strains up to 300% were calculated from measured marker lines displacements and distortion in proximity to the faults. We argue here that the fault-related deformation during fault propagation is manifested by highly localized deformation in a process zone having a width of 0.001–0.005 of the fault length (fault-related deformation due to subsequent slip along the existing faults is analyzed in Part II). The observed self-similarity of the discontinuities over ﬁve length orders of magnitude and the outstanding lack of tensile microcracks suggest fault initiation and growth as primary shear fractures. q 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Faulting; Mechanics; Deformation; Damage; Process zone

1. Introduction and Wiltschko, 1994; Vermilye and Scholz, 1998), and reflects the concentrated local stress field that exists near the Faulting of a rock body is always associated with fault-tip zone (Pollard and Segall, 1987; Reches and deformation of the host rock. This deformation evolves in Lockner, 1994). Further, deformation associated with slip time and is generally distributed in distinct zones along and along existing faults can be attributed to stress concen- across the fault. The present study systematically documents trations developed at irregular features along the faults such fault-related deformation in proximity to small faults within as steps between segments. Slip along irregular faults may a quartz–syenite intrusion and resolves the time evolution generate pull-apart basins (Freund, 1974), or fault-bend- of this deformation. The host rock may be deformed during folds (Suppe, 1985). Fault related deformation may be different stages of faulting: prior to faulting, during fault manifested by microdamage over a large region (Lya- growth, and/or during the slip along an existing fault. Pre- khovsky et al., 1997), small microcracks localized at the faulting deformation is likely to be quasi-uniformly fault tip region (Vermilye and Scholz, 1998), gouge zones distributed in the faulted region and to reflect the of crushed host rocks at the fault core (Chester and Logan, stress/strain fields, that eventually lead to faulting (Aydin 1986), jointing, secondary faults such as Riedel shear and Johnson, 1978; Lyakhovsky et al., 1997). Deformation developed in a wide zone (Aydin and Johnson, 1978), during fault growth is probably concentrated at the fault-tip population of fractures developed locally at fault tips region (Chinnery, 1966; Cowie and Scholz, 1992; Anders (Chinnery, 1966), and fold-like linking between segments

* (Sylvester, 1988). Corresponding author. Correspondence address: Institute of Earth Some patterns of fault-related deformation may be Sciences, Hebrew University, Jerusalem 91904, Israel. Fax: þ972-2- 5380688. attributed to a speciﬁc model of faulting, and several such E-mail address: [email protected] (O. Katz). models are outlined here. (1) Grifﬁth (1924) proposed that a

0191-8141/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0191-8141(03)00011-7 1676 O. Katz et al. / Journal of Structural Geology 25 (2003) 1675–1689 fault nucleates at a critical flaw and it is expected to grow 2. Field analysis of the faults and their associated within its own plane; hence pre-faulting deformation is not deformation predicted by Griffith’s model. Micromechanical observations, however, have showed that rocks contain many 2.1. General setting tensile microcracks that are active before and during faulting (Tapponnier and Brace, 1976; Krantz, 1979). We analyzed small faults within a shallow intrusion in Hence it is generally accepted that growth and interaction Gevanim dome, Ramon area, southern Israel (Baer and of tensile microcracks, not necessarily parallel to the shear Reches, 1989)(Fig. 2). This quartz–syenite body intruded plane, control macroscopic faulting. (2) A few analyses the calcsilicate–carbonate sequence of Triassic age about proposed that a fault grows in the wake of a process zone, a 130 Ma ago (K–Ar whole rock dating by Lang and Steinitz finite size region of high stress concentration at the fault tip (1989)). Several faults divide the intrusion into a few blocks 2 in which the intact rock is disintegrated (Cowie and Scholz, that cover an area of ,1km; the intrusion thickness is at 1992; Reches and Lockner, 1994). These models predict least 100 m. Gevanim quartz–syenite is composed of 5– that fault related damage is concentrated close to the fault 20% quartz, about 5% sanidine and 70–80% albite and and decays away from it. (3) Damage models are based on small amounts of melanocratic phases. Alteration of the assumption that a fault forms due to interaction among feldspar to secondary calcite and cavity-filling calcite is many damage points, mostly microcracks. These micro- common. The studied area is located in the central part of cracks that were generated prior to faulting are assumed to the largest block in the intrusion (Fig. 2), about 20–30 m below the roof of the intrusion and at least 50 m away from coalesce when their density reaches a critical value (Peng its lateral margins. The grain size of the rocks in the studied and Johnson, 1972; Horii and Nemat-Nasser, 1985; Ashby area is 0.19 ^ 0.03 mm and it decreases towards the and Sammis, 1990; Moore and Lockner, 1995; Lyakhovsky intrusion margins. et al., 1997). These models predict that microcracks are not The quartz–syenite intrusion of Gevanim dome is necessarily restricted to the fault tip, and that the density of crosscut by many discontinuities, including several sets of microcracks away from the fault is comparable with the faults, joints, veins and breccia zones. In the present work density of microcracks along the initial fault. we used the word ‘fractures’ for discontinuities that have no The central objective of the present work is to document clear sense of motion, ‘faults’ to indicate shear fractures the deformation associated with natural small faults. We (mode II/III), and ‘joints’ to indicate dilational fractures divide the fault region into three zones (Fig. 1) following (mode I). The most dominant fracture set trends in an N–S Caine et al. (1996): (1) the ‘fault-core’, a narrow zone of direction and it was mapped at 1:500 scale with a total highly disintegrated rock that accommodates a large amount station (EDM) (Fig. 3); the two other fracture sets strike E– of localized shear; (2) the ‘damage-zone’, a wider zone of W and NW–SE. The general nature of these sets is gradually decreasing intensity of the fault related defor- described below. mation; and (3) the ‘protolith’ a region with negligible fault- The N–S set is composed of vertical fractures with strike related deformation. Using this framework, we analyze the that ranges from NNW to NNE (Fig. 3). They are spaced up in-situ deformation associated with three small faults in a to 1 m apart and some of them are tens of meters long (Fig. quartz–syenite body in southern Israel (Fig. 2). The 3a). Some field observations indicate that the fractures of the analyzed discontinuities, which range in length over five N–S set are right-lateral, strike-slip faults: (1) many of them orders of magnitude, reveal deformation structures that are display systematic right-lateral separation of traces of the distributed along the faults within zones of characteristic older E–W fractures (Figs. 4–6); (2) some of them carry width similarly to Fig. 1. We discuss here the timing and horizontal slickenside striations; (3) the deformation style at evolution of the observed fault related deformation. The the links between segments is compatible with right-lateral mechanics of fault nucleation and growth will be presented slip component (see below); and (4) no evidence was found by Katz et al. (in preparation). for dilation across the faults or for dip-slip components. Thus, we regard the N–S fractures as right-lateral, strike-

Fig. 1. Schematic zonation of fault-related deformation along a fault (after Caine et al., 1996). O. Katz et al. / Journal of Structural Geology 25 (2003) 1675–1689 1677

Fig. 2. Simplified geological map of Gevanim Dome, Ramon area, southern Israel (after Zak, 1960); inset is location of Gevanim Dome. Study area (marked by the rectangular frame) is about 20–30 m below the roof of the intrusion and at least 50 m away from its lateral margins. slip faults. Sense and amount of slip on a given fault tinuous joints. Usually these fractures are not dilated and segment were ascertained only when three or more traces of they appear to terminate toward the fractures of either E–W the E–W fractures are similarly displaced. The amount of or N–S sets. These field relations suggest that the SE–NW right-lateral slip on a fault segment varies with the strike of set is younger than the E–W set, and penecontemporaneous this segment (Fig. 3b): slip along NNE striking fault to (or younger than) the N–S set of faults. segment is zero to a few centimeters, slip along N-striking The study now focuses on detailed analysis of three faults fault segments is up to a few tens of centimeters, whereas selected from the N–S fault set. We found that these three slip along a single NNW-striking fault segment exceeds faults represent different stages of fault evolution as 125 cm (Fig. 3). Faults in the NNE direction have narrow manifested by their length, width of the fault-zone, amount fault-zones of a few millimeters width, and they have of slip and segmentation, as well as host rock deformation relatively smooth traces with minor segmentation. Faults in and microstructure. the NNW direction have wider fault zones, up to 15 cm wide, and display crooked traces with intense segmentation. From 2.2. Fault geometry of three right-lateral faults this spatial arrangement with finite slip increasing in faults that have a more westerly strike, it is deduced that this set of strike The three selected faults of the N–S set are marked GF1, slip faults developed under maximal horizontal compressive GF2 and GF3. The exposed trace of GF1 is 15.5 m long and stress trending approximately NE–SW (Fig. 3c). its central portion ,5 m long was mapped at scale of 1:10 The fractures of the E–W set are sub-vertical joints and (Fig. 4a). The map displays four segments with lengths of veins filled with polymetallic sulfo-arsenitic mineralization 0.8–2.0 m, and a general trend of 0–0158 with local (Itamar and Baer, 1993). The filling is a few millimeters to a deviations up to 308 (Fig. 4a). The northern portion consists few centimeters wide with local occurrence of host rock of two parallel segments, 7 cm apart that bound a partially fragments. The E–W fractures commonly form swarms of crushed block. The systematic slip distribution along GF1 several, sub-parallel, curved tensile fractures. The mineral- displays two local maxima of 29 and 40 mm at distances of ization filling the E–W set is 125 ^ 2 Ma old suggesting 5 and 12 m south of its northern tip (Fig. 4b). This slip that these fractures formed in a chilled brittle external part distribution suggests that GF1 is composed of two, linked of the intrusion during emplacement (Itamar and Steinitz, segments that are ,8 m (southern segment) and ,4m 1988). The third, least dominant set of SE–NW-trending (northern segment) long; the link between these segments fractures comprises sub-vertical, relatively short, discon- (marked II in Fig. 4a) shows that they are currently 1678 O. Katz et al. / Journal of Structural Geology 25 (2003) 1675–1689

Fig. 3. The N–S fault set in Gevanim qz–syenite. (a) Fracture map showing dominant N–S faults, as mapped at scale of 1:500 by EDM Total Station teodolite; right-lateral displacements are marked. Areas of detailed mapping on GF1 and on GF3 (Figs. 4 and 6, respectively) are marked. GF2 is located about 50 m west of the mapped area. (b) Individual fault segment displacements versus the segment strike. (c) Length-azimuth, area weighted rose diagram plot showing distribution of all faults in (a) (total cumulative length is 400 m; n ¼ 27); deduced sHMAX direction, the axis of maximal horizontal compressive stress (see text). continuous with one another. The fault-core of GF1 is splays into the dilational quadrant of a right-lateral fault narrow, about 1 mm wide in the north and about 10 mm (Segall and Pollard, 1980). A few fractures with minor wide and calcite-ﬁlled in the south. Breccia was not dilation and up to 5 mm slip appear in an en-e´chelon pattern observed along GF1. at a distance of 1 m north of the fault (Fig. 5a). Bands of GF2 is located about 50 m west of the area mapped in crushed rock that strike 0308 also appear in this area (Fig. Fig. 3. Its trace is about 7.5 m long with a clear northern tip 5a); their structural relations to GF2 are not clear. The fault- and a poorly exposed southern one (Fig. 5a and b). GF2 core of GF2 is 1–20 mm in width, and it contains displays four segments with lengths of 1.2–2.6 m and local brecciated, crushed host rock (see Microstructure below). trends from 3508 to 0108 (Fig. 5a). Right-lateral slip along The steps between the segments with left offset generate GF2 increases from zero at the north tip to an approximately small, up to 20 mm wide, breccia regions (Fig. 5a). constant value of 19.0 ^ 3.0 mm along the rest of the fault The longest mapped fault is GF3 with an exposed trace of (Fig. 5c). The northern end displays abrupt reduction of the 100 m (probably longer) and general trend of NNW (Fig. 3). slip (Fig. 5c), and bending of the fault trace towards 0308 GF3 consists of at least 10 segments with lengths of 2– (Fig. 5a). This bending of GF2 is compatible with fault 38 m, local trends of NNE (0158) to NNW (3408), and slip O. Katz et al. / Journal of Structural Geology 25 (2003) 1675–1689 1679

rock adjacent to the breccia zones is apparently intact. The cumulative slip along GF3 is 125 cm (d–d00; Fig. 6), 65 cm on the western segment (group d–d0; Fig. 6), 45 cm on the eastern segments (d0 –d00), and 15 cm on minor segments inside the central block.

2.3. Distributed damage within the host rock

We used a Schmidt hammer (model 58-C181/F by Controls) to evaluate the in-situ mechanical properties of the host rock in proximity to the selected faults. The Schmidt hammer was originally developed for non- destructive testing of concrete strength. Katz et al. (2000), derived empirical correlations between the hammer rebound and the Young’s modulus, uniaxial strength and bulk density of the tested rocks. They also described the procedures for in-situ, field measurements. In the current study, hammer measurements were conducted along seven profiles (Figs. 4a, 5a and 6), each with 8–17 measurement points, and each point was subjected to tens of hammer impacts (Fig. 7; Table 1). All points were on grinder polished surfaces except profile 4 with manually polished surfaces. The hammer rebound (HR) values in four profiles (1, 2, 5 and 7 in Fig. 7) can be divided into two distinct groups, one of high values and a second of low values; for example, along profile 1, the group of high values is HR ¼ 75 ^ 1, versus the second group of HR ¼ 49 ^ 11. Two other profiles (4 and 6) also show two groups but with smaller differences in rebound values; for example, along profile 6, the high value group has HR ¼ 71 ^ 1 versus the second group of HR ¼ 69 ^ 0. Profile 3 has a constant value of about HR ¼ 72 ^ 1. To quantify the local variations of the rebound values, we convert the hammer readings to Young’s modulus (E), compressive strength (U) and bulk density (D) using our calibration (Katz et al., 2000)(Table 1). We refer to these three parameters as indicating the in- situ ‘competence’ of the rock. The competence variations revealed three noticeable features. First, high competence values are located away Fig. 4. Central portion of Gevanim Fault 1 (GF1) (for location see Fig. 3). from the faults, whereas lower competence values appear at (a) Map, 1:10 mapping scale, displaying fault trace, E–W fractures, sample locations (106–112, 121 and 122) and hammer profiles (legend in Fig. 5); the fault-zones (Fig. 7; Table 1) (with local exception along Roman numbers I–III mark connection points of stretches. (b) Displace- profile 4). A comparison between the background compe- ments along GF1 from offset E–W fractures plotted with respect to distance tence (away from the fault) and the fault-zone competence is from northern fault tip; Roman numbers indicate locations shown in (a). displayed in Fig. 8. The largest competence reduction appears across profiles 1 and 2 of GF1 with a decrease of more than 50% of E and U in the fault zone; the other magnitude of 25–125 cm. A 5-m-long stretch of GF3 was profiles show a corresponding decrease of up to 30% of E mapped at 1:10 scale (Fig. 6); its well-exposed NNW- and U. Second, the fault-normal width of the competence trending segments have the largest observed slip in the reduction zone is scattered (Fig. 7; Table 1), and it is weakly entire studied area. The fault has a left-stepping zone with proportional to the fault length. For example, the width of ,0.5 m offset between en-ećhelon NNW and N–S this zone is ,7 cm for 15.5-m-long GF1 and more than segments. In the mapped stretch, the fault-core consists of 33 cm for the ,100-m-long GF3. Third, the background two major breccia zones that are up to 15 cm wide and a few competence values at the proximity of GF3 are the lowest additional narrow breccia zones. The breccia zones consist background values in the study area (Figs. 7 and 8; Table 1). of pebble-sized fragments cemented by calcite and the host This observation may indicate that the competence 1680 O. Katz et al. / Journal of Structural Geology 25 (2003) 1675–1689

Fig. 5. Gevanim Fault 2 (GF2) (located 50 m west of the area mapped in Fig. 3). (a) Map, 1:10 mapping scale, displaying fault trace, E–W fractures, sample locations (113–119) and hammer proﬁles (see legend); Roman numbers I–III mark connection points of stretches. (b) Picture showing the central part of the fault; note displaced E–W fractures: i and j to i0 and j0, respectively. (c) Displacements along GF2 from offset E–W fractures; plotted with respect to distance from northern fault tip; Roman numbers indicate locations shown in (a).

Table 1 Summary of Schmidt hammer ﬁeld survey of seven proﬁles (locations in Figs. 4–6). The results are presented by mean values of background readings from both sides of the fault and the mean values for the fault zone; W—fault normal distance of competence reduction (Fig. 7). The hammer rebound units (H.R.) were used to evaluated Young’s modulus (E), compressive strength (U) and bulk density (D) according to Katz et al. (2000)

W (cm) H.R. E (GPa) U (MPa) D (kg/m3)

GF1 Profile 1 Background average 10 75 ^ 179^ 4 329 ^ 26 2768 ^ 20 Fault zone minimum 49 ^ 11 21 ^ 18 58 ^ 62 2210 ^ 262 Profile 2 Background average 7 72 ^ 169^ 4 268 ^ 26 2712 ^ 26 Fault zone minimum 55 ^ 631^ 11 89 ^ 42 2371 ^ 132 Profile 3 Background average 0 72 ^ 172^ 4 284 ^ 23 2729 ^ 21 Fault zone average Reading as background GF2 Profile 4 Background average 10 74 ^ 176^ 3 300 ^ 25 2743 ^ 22 Fault zone average 72 ^ 170^ 2 274 ^ 12 2720 ^ 12 Profile 5 Background average 25 73 ^ 173^ 3 292 ^ 16 2736 ^ 15 Fault zone average 69 ^ 063^ 1 230 ^ 6 2672 ^ 7 GF3 Profile 6 Background average 21 , 71 ^ 168^ 2 259 ^ 13 2704 ^ 14 Fault zone average 69 ^ 062^ 0 230 ^ 0 2671 ^ 0 Profile 7 E. background average 33 , 72 ^ 070^ 1 276 ^ 7 2722 ^ 7 W. background average 69 ^ 160^ 2 219 ^ 12 2657 ^ 16 Fault zone average 66 ^ 155^ 1 188 ^ 7 2614 ^ 11

W—Fault normal distance of competence reduction; H.R.—relative hammer rebound units; E—in-situ Young’s modulus; U—in-situ compressive strength; D— in-situ bulk density. O. Katz et al. / Journal of Structural Geology 25 (2003) 1675–1689 1681

Fig. 6. Details of overlap zone between two segments of Gevanim Fault 3 (GF3) (for location see Fig. 3). The map, at 1:10 mapping scale shows the fault trace, E–W fractures, sample locations (36, 37, 40, 65, 84, 90, 123, 124, 309 and 310) and hammer proﬁles (legend in Fig. 5). Note offset of groups of E–W fractures, i.e. group d is displaced 0.65 m to d0 on the western segment and again 0.45 m to d00 on the eastern segment; additional displacement of 0.15 m is distributed between the two segments. reduction zone associated with GF3 is wider than the length of the hammer proﬁles.

2.4. Distortion of lines at the faults’ proximity Fig. 7. Schmidt hammer field profiles showing hammer rebound units along the seven profiles; fault traces are marked by vertical thin lines; profile locations are shown as dashed–dotted lines in Figs. 4–6; field and analysis One striking field observation is the distortion of traces of procedures are specified in the text and in Katz et al. (2000). The arrows some EW fractures in proximity to faults GF2 and GF3. We marked I and II in the lower figure are the width of the fault trace at profiles measured this distortion by first aligning a thin thread to the 6 and 7, respectively. fracture trace on both sides of the fault (dashed line in Fig. 9), and then measuring the deviation of the trace from the 3. Microstructural analysis thread as a function of distance from the fault (v(x)inFig. 9). The distorted traces are assumed to have been linear and A total of 26 vertical cores of 25.4 mm diameter were continuous prior to faulting. We used this technique to collected from the vicinity of the selected faults (locations in measure three traces across GF2 (locations marked with Figs. 4–6; summary in Table 2); microstructural analysis gray arrows in Fig. 5a). For GF3, we used an EDM system was conducted on horizontal, oriented thin-sections pre- (accuracy ^3 cm) to measure a distorted swarm of fractures pared from these cores. The thin-sections were mapped at (location marked with gray arrows in Fig. 6). The results scale of 40:1 by using an optical microscope together with (Fig. 10) show that the line distorted is recognized to scanned computer images. In addition we used SEM images distances of 0.2–0.4 m away from GF2, and 4–6 m away (produced on Jeol-JXA-8600) for 1000:1–200:1 scale from GF3. The structural significance of these measure- examination of the microstructures. ments and their implications to fault evolution are analyzed We found four types of microstructures: fault-oblique in Part II (Katz and Reches, in review). microfractures, fault-parallel microfractures, calcite-filled- 1682 O. Katz et al. / Journal of Structural Geology 25 (2003) 1675–1689

The microstructures are described in detail below for the fault core and the damage-zone.

3.1. Fault-core

The microstructure of fault-cores was analyzed in thin sections made from 10 samples drilled into the cores of the three studied faults (Figs. 4–6; Table 2). In the field, GF1 displays a quasi-planar, thin zone of varying width (0.1– 10 mm) and with no breccia. In thin-sections, the internal part of the core consists of opaque oxide (Fig. 11a–d), quartz cement of 10–50 mm grain-size (Fig. 11a and b) and brown, green or clear calcite that fills thin veins, which cut the oxide (II in Fig. 11d). Clear calcite cement also fills an irregular network of interconnected fractures and cavities (Fig. 11a–d). A few rock fragments appear along the core (a in Fig 11c and d). In the field, the fault-core of GF2 is up to a few tens of millimeters wide with few bands of micro-breccia (fragments are up to 0.2 mm). In thin-sections, the fault-core of GF2 (Fig.12a and b) consists of several quasi-linear zones of micro-breccia that strike 345–0158. Each zone is 0.25– Fig. 8. Comparison of background versus fault-related competence 1.25 mm wide and contains angular rock fragments of the parameters according to Schmidt hammer analysis along seven profiles (Fig. 7; Table 1). (a) Young’s modulus, E; (b) Compressive strength, U. The quartz–syenite (a in Fig. 12a and b) cemented by brown, diagonal lines show the marked percentage of competence reduction close greenish-brown (b in Fig. 12a and b) and clear calcite (b1in to the fault. Fig. 12b). A few E–W-striking fractures are cut and displaced by the brecciated zones (IV in Fig. 12a and b). networks and breccia zones; all will be described in detail In the field, the fault-core of GF3 is an about 100-mm- below. The fault-oblique microfractures were found in all wide breccia. In thin-sections, the breccia is cemented by samples regardless of fault-distance, and are regarded by us brown calcite, which includes a system of 0.5–1.5-mm- as unrelated to the dominant N–S faulting. The three other wide bands of poorly sorted angular fragments, 0.01– types of microstructures display systematic distribution with 0.40 mm in size (gouge?) (a in Fig. 12c). Narrow zones of respect to the damage zonation. We found no intragranular, host rock fragments appear between these bands (g in Fig. tensile microfractures that can be related to the N–S faults; 12c). The fragments in the bands indicate a mixture of this remarkable lack of intragranular, tensile microcracks is opening, shear and rotation modes. Fault parallel (NNW– in agreement with our triaxial experiments on Gevanim NNE) breccia bands cemented by clear calcite locally cut quartz–syenite samples (Katz and Reches, 2000). The the GF3 main breccia. This clear calcite breccia consists of implications of this observation will be analyzed in a angular rock fragments up to 1 mm in size. following manuscript (Katz and Reches, in preparation). In summary, the studied three faults are characterized by an abundance of cementation by secondary quartz and calcite in networks and veins, dominance of micro-breccia and negligible amount of gouge.

3.2. Damage-zone

Two types of microstructures dominate the damage-zone (Fig. 1): calcite-filled-networks and microfractures. A calcite-filled-network consists of inter-connected small cavities filled with light-yellow calcite locally rimmed with brown calcite. The cavities are less than 0.01 mm in Fig. 9. Schematic presentation of the distortion of linear fracture traces in size and they are associated with intragranular and short proximity to GF2 and GF3. Heavy N–S line—the fault; thick, curved intergranular fractures. Most of the fractures in the calcite- lines—the E–W-trending fractures distorted near the faults; dashed E–W filled-networks are aligned in a N–S direction (d in Fig. 12a lines—linear fracture traces prior to faulting. The coordinate system of deformation analysis: x, y—fault normal and fault parallel axes; u, v— and b). Larger, filled voids (.1 mm) may also be part of a corresponding displacements; [v(x)]—a distorted line that was initially calcite-filled-network (Fig. 11a and b). normal to the fault; 2W—width of distortion zone. Microfractures are by far the most dominant structures in O. Katz et al. / Journal of Structural Geology 25 (2003) 1675–1689 1683

Fig. 10. Line distortion across GF2 and GF3; coordinates defined in Fig. 9, dashed horizontal line in the x coordinate is a reference line. (a) Three field- measured profiles of fracture traces across GF2 (locations in Fig. 5); shown fault-parallel line displacement, v(x), with respect to initial position. (b) A distortion profile measured across GF3 (location in Fig. 6); diamonds are points measured on a E–W fracture swarm with EDM total-station. Vertical exaggeration of v(x) is the same in (a) and (b). the thin-sections. They display a wide range of properties: of NNW to NNE (Fig. 15a) and a fault-oblique group that intergranular and transgranular, length range from ,1to strikes mainly in the range NW to SW (Fig 15b and c). The .25 mm, width range of 0.01–1.0 mm, and orientations few observed crosscutting relations indicate that the fault- and crosscutting relations that vary considerably. Most of parallel microfractures and microfaults cut and displace the the microfractures are filled with secondary minerals (Fig. fault-oblique microfractures; for example, truncation and 13) and their composition was studied by the electron apparent displacement at point IV in Fig. 12a and b. These microprobe. We found clear calcite (massive or granular), relations suggest that the fault-parallel microfractures are dark brown calcite with iron oxides and greenish brown younger. calcite with clays. Rock fragments within the fractures are The crosscutting relations of the fracture fillings indicate enclosed in brown calcite (a in Fig. 13a). Less abundant the following relative ages (from early to late): (a) opaque fracture fillings are fine-grain quartz and opaque black or oxides (crosscut by fine-grain quartz, I in Fig. 11b; brown brown oxide. The amount of slip along the microfaults calcite, II in Fig. 11d; and clear calcite, III in Fig. 11b and varies from 0.1 to 5 mm according to the displaced crystals d); (b) fine-grain quartz (crosscut by brown calcite, IIa in (Fig. 14). The microfractures commonly display en-ećhelon Fig. 13b; and clear calcite, III in Fig. 11b and IIIa in Fig. patterns, including pull-apart openings (Fig. 14). Displace- 13b); (c) brown (and greenish brown) calcite (crosscut by ments are dextral in most cases (eight out of nine observed clear calcite, IIIb in Fig. 11d and IIIb in Fig. 13b); (d) clear micro-faults) although sinistral displacements were also calcite (Figs. 11a–d and 13a and b). The opaque oxides observed. The microfaults with evident shear displacement belong to the crystallization phase of the quartz–syenite consist of rock fragments enclosed in brown calcite. intrusion (Early Cretaceous; Itamar and Steinitz, 1988). The The microfractures can be divided on the basis of their age of the fine-grain quartz is unknown. The (c) and (d) direction into a fault-parallel group that strikes in the range fillings appear between angular rock fragments and veins (a 1684 O. Katz et al. / Journal of Structural Geology 25 (2003) 1675–1689

Table 2 Summary of microstructural analysis of 26 thin-sections from samples collected across the three studied faults in Gevanim. Sample locations are shown in Figs. 4–6. Samples 108, 110, 121, 117 and 118 include the fault core and additional area of about 1 cm at its proximity

Sample (#) Distance from fault core (cm) Microstructures

Fracture direction Fracture ﬁlla Otherb

GF1 108, 110, 121 0, 0, 0 N (360–0158) OX, FQ, BC, PC CC, IC NBC NW OX, FQ, BC, PC NE BC(þRF), PC CC, IC 106, 111 2, 2 W OX, FQ NE OX, PC 109 9 N PC N–NE BC, PC W BC(þRF), PC 122 15 WSW FQ 107, 112 30, 35 NNW BC, PC NE BC GF2 117, 118 0, 0 N (345–0158) GC, BC, PC, RF CC 116 4 N, NNEc BC(þRF), PC IC, CC W BC(þRF), PC IC, CC NW PC Intergranular PC Rare 120 2 N of the tip NNWc –NNE BC(þRF), PC, FQ E–NEc BC(þRF), PC CC WNW FQ 115 11 NWc BC(þRF), PC NE BC(þRF), PC NPC 113,119 45, 23 NW BC(þRF), PC NE BC, PC GF3 309, 310 Breccia zone No directional data BC(þRF) IC 36, 40 4, 4 NNE BC(þRF), PC CC, IC W–N BC(þRF) CC, IC 124 4 non (breccia) BC(þRF) CC, IC NNW–NNE PC(þRF) W BC(þRF), PC 123 13 NNWc –Nc BC(þRF), PC IC NE–E OX, PC 37, 90 18, 28 NWc –N BC(þRF), PC CC, IC NE BC(þRF), PC 84 72 NNE PC, BC CC, IC WPC 65 85 Nc PC, BC IC NW BC, PC

a B/G/PC—brown/green/clear calcite; FQ—fine grain quartz; RF—rock fragments; OX—Opaque oxides. b I/CC—intergranular/cavity fill calcite in the fractures and the host rock. c Evidence for shear displacement on fractures of this direction. in Figs. 12 and 13) that are related to slip and dilation along associated fault. The nature of the deformation is described the fractures. below in terms of the zonation defined in Fig. 1.

4. Zonation of fault-related deformation 4.1. Fault-core

Our field and microstructural analyses cover faults with The fault-core is the zone that accommodates the dimensions spanning over five orders of magnitude, from displacement between the two blocks of the fault (Chester microfaults of sub-millimeter length, to meter long faults et al., 1993). We identified the fault-core in the field as a (GF1 and GF2), and hundred meter long GF3. Fig. 16 dark, featureless band (GF1), or as a (micro) breccia-zone presents the widths of the zones of the observed deformation (GF2 and GF3). The displaced fractures of the E–W set structures across the analyzed set of faults. The widths of the could not be recognized inside the fault core due to zones are apparently proportional to the length L of the obliteration by the localized shear. The simple shear strain O. Katz et al. / Journal of Structural Geology 25 (2003) 1675–1689 1685

Fig. 11. Photomicrographs and maps of samples 108 ((a) and (b)) and 121 ((c)–(d)) at GF1 (locations shown in Fig. 4), showing the crosscutting relations of the fracture fillings. Opaque oxides are crosscut by fine-grain quartz (I at b), brown calcite (II at d) and clear calcite (III at b and d); fine- grain quartz is crosscut by clear calcite (III at b); brown (and greenish Fig. 12. Microstructure of GF2 (sample 117) and GF3 (sample 310), see brown) calcite are crosscut by clear calcite (IIIb at d); Few rock fragments Figs. 5, 6 and 15a for locations; legend in Fig. 11. (a) Photomicrograph and appear along the core (a at c, d). (b) map of GF2 core; it is brecciated with a few, partly linked bands in a in the fault-core is: generally north direction that contain angular rock fragments (marked a in (a) and (b)) in a matrix of brown, greenish-brown (b in (a) and (b)) or clear = g ¼ S 2WC calcite (b1 in (b)); large blocks of intact rock appear in the matrix between the zones (marked g in (a) and (b)); a few E–W-striking fractures are cut where S is fault slip and WC is half the fault-core width as and displaced by the breccia zones (IV in (a) and (b)); note a fault-parallel measured in the field. The shear strain is about 350% across fracture in the upper-right (d in (a) and (b)). (c) Photomicrograph of GF3 core which is a breccia zone with elongated bands of angular, poorly sorted GF1 core (S , 35 mm, 2WC , 10 mm), 200% across GF2 rock fragments (center of the photomicrograph, marked a) cemented by core (S , 20 mm, 2WC , 10 mm) and up to 500% across 00 brown calcite; large blocks of intact host rock occur between the zones GF3 (S ¼ 125 cm, 2WC ¼ 25 cm; d–d area in Fig. 6). This (marked g). localized shear leads to intense host rock deformation manifested by brecciaed rock fragments and closely spaced intensity of fault-related deformation gradually decreases microfractures (Figs. 11 and 12). The visible fault-core outward (Chester et al., 1993). The damage-zone of the width in the field is ,0.001L; the width is larger, 0.002– studied faults includes several deformation features. First, 0.004L, for microfaults and small faults in triaxial tests (Fig. calcite-filled-networks (Fig. 11) and fault-parallel micro- 16). fractures (Figs. 12 and 15) that occur in zones with fault normal widths of 0.005L and 0.02L, respectively (Fig. 16). 4.2. Damage-zone Second, competence reduction of the host rocks as revealed with the Schmidt hammer takes place over a zone up to This is a wide zone of damaged rock in which the ,0.03L wide (Figs. 7 and 8). We suspect that the width of 1686 O. Katz et al. / Journal of Structural Geology 25 (2003) 1675–1689

the reduction zone near GF3 is an underestimation of the real width because the background Young modulus close to GF3 is lower by 6–7% than the equivalent close to the other two faults (Table 1). Third, distortion of fracture traces that vanish at a distance of 0.2–0.4 m away from GF2, and 4– 6 m away from GF3 (Fig. 10); these distances are about 0.05–0.06 of the corresponding fault length (solid dots in Fig. 16).

4.3. Protolith

This is a region with negligible fault-related deformation, which extends outwards from the external limit of the damage-zone. The Schmidt hammer readings return to the background levels of the undeformed host rock.

Fig. 13. (a) Photomicrograph and (b) map showing secondary minerals in a microfracture (sample 120, tip of GF2; for locations see Figs. 5 and 15; 5. Discussion legend in Fig. 11). The minerals include clear calcite with massive and granular appearances; brown calcite in the fracture rims or center and fine- grain quartz; the brown calcite crosscuts the fine-grain quartz (IIa) and clear 5.1. The process zone of brittle faulting calcite crosscuts both of them (IIIa and IIIb, respectively); the brown calcite in the fracture center contains rock fragments (a in a) that lack in the clear The concept of ‘process zone’ was introduced to calcite fill. incorporate the non-linear character of a fracture tip into linear elastic fracture mechanics framework (Lawn, 1993). Irwin (1958) divided the crack system into a linear-elastic outer zone that transmits the applied loading to the very small zone (process zone) around the fracture tip. The stresses in the process zone reflect the stress concentration and stress field at the fault tip (Lawn, 1993). Barenblatt (1962) and Dugdale (1960) described fracturing through a cohesive, plastic, process zone that is located in the propagating fracture tip in which energy absorption processes may operate. In brittle rocks, these energy absorption processes are primarily manifested by microcracking of the intact rock (Cowie and Scholz, 1992; Reches and Lockner, 1994). In the field, the dimensions of the process zone were determined by the occurrence of tensile microfractures (Anders and Wiltschko, 1994; Vermilye and Scholz, 1998) and large fractures (Little, 1995). It was shown in several field cases that the process zone width is about 0.01 the fault length (Cowie and Shipton, 1998; Vermilye and Scholz, 1998). Scholz et al. (1993) further used the Dugdale–Barenblatt concept and estimated that the process zone length scales with the process zone width. Another approach considers a constant value for the process zone width that is controlled by the stress intensity at the fault tip and by the mechanical properties of the host rock (Reches and Lockner, 1994; Lyakhovsky, 2001). Fig. 14. Microfaults mapped on a thin section and SEM images (sample What is the nature of the process zone in Gevanim faults? 115, location shown in Fig. 5). (a) and (b) Segmented, dextral microfaults Previous field studies used the abundant tensile microcracks with en-ećhelon geometry including micro pull-aparts filled with brown to delimit the process zone (Anders and Wiltschko, 1994; calcite and brecciated rock fragments; sheared quartz crystals reveal Vermilye and Scholz, 1998). In our field area, however, the displacements of ,0.25 mm (locations are shown in Fig. 15). (c) SEM faults developed with negligible amounts of tensile micro- image of the microfault in (b); Q, F and Px are quartz, feldspar and pyroxene (aegirine) crystals and F.C. is fault core; note the extremely fine cracks (see above), and a different criterion is needed to breccia inside F.C. (d) Close-up of a single quartz grain adjacent to the define the process zone dimensions. We examine the two microfault in (c); note the lack of dilational microcracks. zones described above, the fault-core and the damage-zone O. Katz et al. / Journal of Structural Geology 25 (2003) 1675–1689 1687

Fig. 15. Microstructural maps of GF2 samples (upper row) and corresponding fracture rose diagrams below (length-azimuth, area weighted); mapping at 40:1 scale of scanned images and an optical microscope; (a)–(c) samples 117, 115 and 113 across the fault (distances are marked), and (d) sample 120 NE from the northern tip area (locations of samples are shown in Fig. 5b). The areas mapped in detail and shown in Figs. 12a, 13a and 14a and b are marked. In gray are unmapable notches on the sections surface used to orientation inference. (see Section 4). The fault-core contains breccia (Figs. parallel shear microfractures and calcite-ﬁlled networks but 11–13) and underwent large simple shear (200–500%), and it retains its physical coherence. Thus, it is unlikely that the it is a region of breakdown and disintegration of the intact damage-zone is part of the process zone. host rock. Thus, the fault-core is likely to be considered as The present interpretation restricts the use of the ‘process part of the process zone; a portion of the simple shear could zone’ to the region of breakdown, disintegration and reﬂect the slip that post-dated the fault formation. The extreme strain, which in the present study corresponds to situation is different with the damage-zone. It contains fault- the fault-core. The process zone width according to this interpretation is smaller than the one recognized in previous works; it is 0.001–0.005L in the present work (Fig. 16) whereas it is about 0.01L in Anders and Wiltschko (1994) and Vermilye and Scholz (1998).

5.2. Evolution of host rock deformation along Gevanim faults

In this section we discuss the evolution of the fault- related deformation of the studied faults by outlining the pre-faulting stage and faulting stage; the post-faulting deformation is analyzed in Part II. It is anticipated that pre-faulting deformation will display a quasi-uniform distribution (Reches and Lockner, 1994; Lyakhovsky et al., 1997). Our observations, however, of rock brecciation (Fig. 12), reduction of rock competence (Fig. 8), and the distribution of shear and tensile microfractures (Fig. 15) are all restricted to a zone narrower than ,0.03 the fault Fig. 16. Zonation of host rock deformation across faults in Gevanim quartz–syenite. Shown maximum fault normal width (W, defined in Fig. 9) length (Fig. 16). The restriction of these features to the of the marked structural features versus length of the associated faults (L) faults proximity indicates that they are not pre-faulting (see text and Table 2). Micro—indicates observations along microfaults in structures. Further, it is also commonly accepted that tensile thin-sections (Figs. 14 and 15); Lab—indicates faults in Gevanim quartz– microfractures dominate the pre-failure damage of brittle syenite that developed in triaxial failure tests (Katz, 2002); field data is rocks (Peng and Johnson, 1972; Hadley, 1976; Horii and marked with host fault name (GF1, GF2, GF3); DSR—a dashed line indicating the range limit of the field survey for fault related deformation; Nemat-Nasser, 1985; Reches and Lockner, 1994). The note that the width of the calcite-network and the fault-parallel outstanding lack of tensile microfractures in Gevanim microfractures across GF3 is bounded by the sampling distance. quartz–syenite (see Section 3) indicates that the studied 1688 O. Katz et al. / Journal of Structural Geology 25 (2003) 1675–1689 faults formed by a mechanism that is not based on tensile GF3 (Figs. 4–6). Part of the irregular shape is a microfracturing. manifestation of the segmented nature of the faults, starting We suggest that the faults in the quartz–syenite grew as from 1–10-mm-long faults (Fig. 14) to a maximum primary shear fractures based on observations of shear recognized length of an individual segment of about 1 m fractures over wide range of sizes (Figs. 4–6 and 11–13). (GF1 and GF2 in Figs. 4 and 5). These fractures revealed striking geometric similarities Fault-related deformation. Deformation indicators regardless of scale: (1) the shear fractures are composed of include fault displacement, breccia zones, fault-related linked segments (Figs. 4–6, 14 and 15); (2) the segments are microstructures, distortion of fracture traces and compe- slightly crooked to quasi-linear (Figs. 4–6 and 14); and (3) tence reduction. Each of these indicators is distributed along tensile microfractures that are related to the faults are the associated faults within a zone of characteristic width practically missing down to scale of 5 mm(Fig. 14d). This (Fig. 16). self-similarity suggests that the faults grew by the same Timing of fault-related deformation. Fault-related defor- mechanism from the intergranular micro-shears (Fig. 14)to mation was formed during two main episodes: (1) during the 100-m-long fault (Fig. 3). We envision that this growth fault propagation when highly localized deformation occurred first by lengthening of the intragranular micro- occurred within the process zone. This deformation is shears in their own plane, followed by their coalescence restricted to the fault core with width of 0.001–0.005 of with other micro-shears to form the segmented micro-fault fault length; (2) post-faulting deformation of the host rock (Reches, 1987, 1988)(Fig. 14). This sequential process of that is analyzed in Part II. One noticeable result is that none lengthening and coalescence continued and eventually of the deformation indicators could be clearly attributed to forms faults of a meter length and more. Finally, the the pre-faulting stage of distributed damage. observed self-similarity suggests that the three mapped Fault growth mechanism. We observed self-similarity of faults represent different stages of fault development. Faults the discontinuities over five length orders-of-magnitude and GF1 and GF2 are relatively ‘young’ with minor amounts of an outstanding lack of tensile microcracks. These obser- breccia and thin fault-core, whereas GF3 is already in a vations suggest that fault growth occurred in shear, mode ‘mature’ stage. II/III, without contribution of tensile fracturing (Katz and During the faulting stage, the propagating faults Reches, 2000). Evidence and implications of this mechan- generated highly localized deformation in the process ism are the subject of future research (Katz et al., in zone manifested primarily as micro-breccia and high shear preparation). in the fault-core (Fig. 12). The scarcity of gouge material (Figs. 11 and 12) indicates that continuous wear along the faults was probably negligible. Acknowledgements The distorted fracture traces presented above (Fig. 15)is interpreted by us as indicating continuous post-faulting The fieldwork accomplished with the invaluable techni- deformation of the host rock. The analysis and justification cal support of Ya’akov Refael and Shlomo Ashkenazi from for this conclusion are presented in Part II. the Geological Survey of Israel. Discussions with Vladimir Lyakhovsky significantly contributed to the quality of this study. The critical reviews of Jan M. Vermilye and an 6. Concluding remarks anonymous reviewer significantly improved the manuscript and their comments are greatly appreciated. Thanks to Rod The structural analysis of the quartz–syenite body in Holcombe for the permit to use the GeOrient program. The Gevanim reveals the following characteristics of the faults study was partially supported by the US–Israel Binational and their associated deformation. Scientific Foundation grant 98-135, and by the Geological Fault dimensions. The dimensions of the studied Survey of Israel project 30255. fractures span over five orders of magnitude (Fig. 16). The spacing of the faults is not uniform. The fault parallel microfaults, 1–10 mm long, are restricted to zones along the References longer faults (Figs. 15 and 16). The spacing of the medium- size faults, 1–10 m in length, is at approximately the fault Anders, M.H., Wiltschko, D.V., 1994. Microfracturing, paleostress and the length, 1–5 m (Fig. 3). The 100-m-long fault is unique growth of faults. Journal of Structural Geology 16, 795–815. 2 within the ,0.3 km study area (Figs. 2a and 3). A Ashby, M.F., Sammis, C.G., 1990. The damage mechanics of brittle solids noticeable negative observation is the scarcity of tensile in compression. Pure and Applied Geophysics 133, 489–521. microcracks in the analyzed rocks. Aydin, A., Johnson, A.M., 1978. Development of faults as zones of Geometry of fault traces. The mapped faults display deformation bands and as slip surfaces in sandstone. Pure and Applied Geophysics 116, 931–942. fairly continuous traces that are curved and irregular from Baer, G., Reches, Z., 1989. Doming mechanisms and structural develop- the ,1 mm scale (Fig. 14c), through the microfaults ment of two domes in Ramon, Southern Israel. Tectonophysics 166, (Fig.14a and b) to the field scale faults of GF1, GF2 and 293–315. O. Katz et al. / Journal of Structural Geology 25 (2003) 1675–1689 1689

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